Leverage Air Barriers to Manage Air Leakage and Moisture

Air barriers are often considered purely as an efficiency measure to control energy losses associated with air-leakage to or from the building’s conditioned spaces. In reality, air barriers serve energy efficiency and moisture control functions. In fact, movement of moisture-laden air into well insulated building assemblies can cause much more condensation or moisture accumulation to occur within the assembly than might otherwise result from water vapor diffusion alone. Controlling water vapor diffusion with a vapor retarder is practically futile without also controlling air leakage with an air barrier. Thus, air barriers deserve special attention in design and construction from a moisture durability perspective.

Going beyond code minimum practices can provide significant benefits in energy conservation and moisture control. For example, Table 4–10 provides important locations for air barriers intended to manage moisture and uphold the performance of the insulation based on the EPA’s ENERGY STAR Homes program. These may exceed minimum code requirements for only a single air barrier location on the interior or exterior side of an assembly. For example, in Climate Zones 4–8 an air barrier is required on the interior and exterior side of wall insulation for an exterior wall assembly. Similarly, air barriers are required at interior horizontal surfaces and exterior perimeter vertical surfaces of insulation for floors in all climate zones. Air impermeable insulation materials such as closed-cell spray foam can serve the purpose of insulation as well as air barrier, and may be considered as an alternative to the recommendations in Table 4–10 which presumes the use of air-permeable cavity insulation.

Figure 4–16 shows one example of an air barrier installation on the interior side of an exterior wall and another on the exterior side. In both cases the air barrier is continuous with the ceiling at the roof level. Where an interior and exterior air barrier is installed, both of these practices are used as a means for improved insulation performance (energy efficiency) and air leakage control (water vapor control and energy efficiency).

A practical interior air barrier installation involves detailing the interior gypsum board finish as an air barrier layer along with framing connections between walls. A typical exterior air barrier involves the use of a water-resistive barrier membrane (e.g., wrap) with taped joints and penetrations or an approved exterior panel type material, also with taped or sealed joints and penetrations. The two methods shown in Figure 4–16 are common examples. Whichever interior and/or exterior air barrier method and material is used must be coordinated with the vapor control strategy for the assembly (Section 4.3.3) and the rainwater management approach for exterior air barrier applications (Section 4.2).

While there are a variety of air barrier materials worth considering and integrating into the overall moisture control strategy for a building envelope, these materials all must comply with building code and certain material or assembly testing requirements for air permeance. Of course, some materials like wood structural panels, gypsum board, concrete, and sheet aluminum or polyethylene, have obvious air barrier material characteristics. Others may depend on characteristics that are not easily observed by the naked eye. Thus, manufacturer data on air permeance (and also other properties such as vapor permeance) should be sought and used as a basis for design and material selection.

It is always necessary to use air barrier materials in a way that creates a continuous air barrier layer to prevent air leakage through a multitude of potential leakage points (Figure 4–17). Building codes and good practice include requirements to seal all joints and penetrations for visual confirmation or, alternatively, require air pressure testing of the building (e.g., “blower door” test) to verify performance of the final air barrier installation. This course recommends use of a blower door test. It will measure the air change per hour (ACH) of a building to ensure energy code compliance and is a useful diagnostic and training tool (with the use of a smoke stick) to help locate and correct air leaks that may otherwise go undetected and cause potential moisture durability problems within assemblies.